The present invention relates to an apparatus for dewatering sludge, more particularly to an apparatus for dewatering sludge pieces coated with dewatering powders under isostatic pressure.
Sludge is generated by sewage treatment, industrial waste water treatment, etc. in large quantities. Sludge recovered from treatment facilities such as sewage sedimentation ponds usually contain more than 90% by weight of water. Since the sludge is usually dumped or burned, it should be dewatered as much as possible. Particularly in order that it is burned without requiring much fuel, or in order that it is self-burning, the sludge should have a water content of less than about 60 weight %, preferably less than 50 weight %.
Generally, sludge is agglomerated by adding thereto inorganic modifiers such as slaked lime, iron chloride, or organic modifiers such as polymeric agglomerating agents. Agglomerated sludge is then dewatered by a vacuum dehydrator, a centrifugal dehydrator, a belt pres, a filter press, etc., thereby solidifying it. The sludge thus dewatered and solidified may be called "primary dewatered sludge." The primary dewatered sludge usually has a water content of about 70 weight % or more, for instance, about 70-85 weight % particularly when dewatered by a belt press or a filter press, etc.
It was found that pressing the primary dewatered sludge hardly reduces the water content less than 70 weight %. In addition, pressing it under a high pressure is likely to cause the clogging of a filter cloth, unabling further reduction of the water content
European Patent Laid-Open No. 0092146A2 discloses a process for dewatering sludge which comprises the steps of disintegrating the primary dewatered sludge to form sludge pieces while preventing the disintegrated sludge pieces from becoming like a slurry, conditioning the disintegrated sludge pieces by coating them with dewatering powders, and compressing the conditioned sludge pieces to force water to ooze out of the sludge pieces, thereby reducing the water content less than 60 weight %.
FIG. 16 specifically shows a system for carrying out the process of the above European laid-open application. Mud containing organic materials and having a water content of about 95%, as precipitated on the bottom of a water path, a river, a precipitation pond, etc., is admixed with a polymeric agglomerating agent. The mud precipitated and concentrated by the addition of the agglomerating agent is supplied to a belt press A shown in FIG. 16 and is pressed between filter cloths on the belt press for primary dewatering. A sheet of primary dewatered sludge having a thickness of about 20 mm or less and a water content of about 70-85% is obtained. The primary dewatered sludge then is supplied to a disintegrating machine B, which comprises a rotatable cylindrical drum having small plates or rods projecting from the inner wall thereof and a rotatable shaft having disintegrating blades provided at a lower location in the drum. Dewatering powders are introduced into the disintegrating machine B. The dewatering powders may include incineration ashes which may be provided from a multistage incinerator F. The ashes may be, for instance, about 10-25 weight % of the primary dewatered sludge on a wet basis. The disintegrated sludge is coated with the dewatering powders charged into the disintegrating machine B, which process may be called "conditioning."
The operation time of the disintegrating machine is about 20-40 seconds. However, if continuous disintegration and conditioning are desired, the disintegrating machine is constructed such that it is charged with them continuously and retains them for about 20-40 seconds, during which the disintegration and conditioning take place simultaneously. The thus conditioned sludge are such that at least about 70% by weight thereof have a size of about 10 mm or less and their surfaces are substantially coated with incineration ashes. They are not substantially kneaded so that their inner structures are not substantially destroyed.
The conditioned sludge pieces thus prepared in the disintegrating machine B are then supplied to a press C. The press C has a ram connected to a piston of a hydraulic cylinder for compressing the sludge by moving the piston downwardly. A thick, perforated plate having a large number of pores is provided on the bed of the press, and a filter cloth is laid on the bed. The conditioned sludge pieces are placed on the filter cloth. A thick perforated plate is mounted on the lower end of the ram, and a filter cloth is provided on the lower side of the perforated plate. A batch of the conditioned sludge pieces is compressed between the press bed and the ram in the press C. A filtrate produced by the compression is sucked out by a vacuum pump through the pores of the upper and lower perforated plates.
Next, the disc-shaped sludge is disintegrated in a disintegrator D, and then is introduced into a multi-stage sludge incinerator F through a conveyor E. A portion of ashes produced by burning the sludge in the incinerator F may be supplied to the disintegrating machine B through a pneumatic pipe. The remainder of the ashes is stored in an ash hopper G for subsequent disposal.
It has been found, however, that the pressing of the primary dewatered sludge pieces conditioned with dry dewatering powders for further dewatering is not always effective as a secondary dewatering step, because the sludge pieces poured into the press C tend to have an uneven top surface, making it likely to be compressed ununiformly. Specifically speaking, the sludge pieces are usually charged ununiformly in the press and the bottom surface of the ram is flat, so that uneven pressure is applied to the sludge pieces contained in the press from position to position. In addition, the sludge pieces subjected to higher pressure too quickly are less likely to enjoy efficient dewatering because their inner structures are destroyed as mentioned later, so that the total dewatering efficiency rather decreases. It has also been found that such phenomenon is peculiar to sludge pieces which retain water paths or capillaries in their inner structures.